Thermoelectric based generators have been used successfully and reliably for the past 40 years to power deep space probes. These solid-state devices rely only on a temperature gradient to produce electricity by way of the Seebeck effect. The efficiency of a thermoelectric device is directly related to the ΔT, hot junction temperature, and the thermoelectric figure of merit, zT=S2T/ρk, where S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the total thermal conductivity, and T is the absolute temperature. These properties are interconnected and improvement in one property often leads to diminishment of another. As a result, the zT for most thermoelectric materials has historically remained relatively low, limiting thermoelectric devices to niche applications such as in space.
Recent advances in thermoelectric materials research have led to higher efficiency materials with roughly a twofold improvement in energy conversion efficiency over legacy materials such as Si1-xGex and PbTe. Unlike such traditional materials, however, modern high-efficiency thermoelectric materials tend to behave as weak and brittle ceramics. The fragility of these materials increases the complexity of machining, lowers the yield, and constrains potential device configurations. All of these factors add to the cost and difficulties in developing functional thermoelectric devices. Moreover, there can be significant thermal stresses during operation as the temperature differential can often be several hundred degrees between the hot and cold junction of a thermoelectric device. Poor mechanical properties result in a low tolerance to such thermomechanical stresses encountered during operation.
Early work unrelated to thermoelectric research investigated the impact of introducing transition metals into brittle cemented carbides. For instance, Bolton and Keely confirmed that the Co inclusions in WC significantly increased the facture toughness of the carbide (J. D. Bolton and R. J. Keely, Fibre Science and Technology, 1983, 19, 37-58). The ductility of Co allows for plastic deformation that inhibits the propagation of fractures while undergoing tensile stresses. However, in the case of thermoelectric materials, the introduction of transition metals presents a number of problems. Such metals are generally associated with high thermal conductivities and relatively low Seebeck coefficient values. Consequently, the introduction of metal inclusions into a thermoelectric material would be expected to result in a composite with a lower thermoelectric figure of merit thereby reducing efficiency.
What is needed is a composite thermoelectric material having improved mechanical properties without compromising thermoelectric efficiency, and a method of making such a material. Surprisingly, the present invention meets this and other needs.